FIG. 12 is a diagram illustrating an air-conditioning system. As shown, an air conditioner 64 includes a compressor (not shown) for compressing a gaseous refrigerant, a condenser 66 for cooling and condensing, by using cooling water, the gaseous refrigerant that has generated heat upon being compressed, and an evaporator (not shown) for evaporating the refrigerant by permitting it to flow in through an expansion valve, the refrigerant being obtained by condensation through the condenser 66.
The condenser 66 is provided in a cooling tank 70 to cool the gaseous refrigerant that flows through the condenser 66, and cooling water is fed to the cooling tank 70 from a cooling tower 68. The cooling tower 68 feeds the cooling water to the cooling tank 70, and includes a cylindrical tower body 72 and a water-receiving vessel 74 provided under the tower body 72, the water-receiving vessel 74 and the cooling tank 70 being connected together through a feed line 76.
The tower body 72 incorporates therein a filler unit 78 having many passages though which the cooling water and the cooling air flow. The tower body 72 has a spray nozzle 80 for spraying the cooling water onto the filler unit 78, the spray nozzle 80 being connected to the cooling tank 70 through a return line 82, whereby the cooling water in the cooling tank 70 is fed to the spray nozzle 80 by a circulating pump 84 provided in the feed line 76.
The cooling water sprayed onto the filler unit 78 from the spray nozzle 80 flows through many passages formed in the filler unit 78, and falls into the water-receiving vessel 74. Thus, a cooling water path through which the water circulates is formed by the cooling tower 68, cooling tank 70, and feed line 76 and return line 82 connecting them together, and the water flows through the cooling water path by operating the circulating pump 84.
A blower 86 is provided at an upper position in the tower body 72, the air flows in being sucked up from the lower portion of the tower body 72 by the blower 86, and the air that has flowed in flows through the passages in the filler unit 78 against the flow of the cooling water. The cooling water comes into direct contact with the air that flows reversely, and partly evaporates while exchanging heat. The cooling water is further cooled by losing evaporation heat. To replenish the cooling water that has decreased by the evaporation of the cooling water, the tower body 72 is replenished with the cooling water through a replenishing line 90 that may be opened or closed by a float 88.
As described above, the cooling tower 68 cools the cooling water by utilizing the loss of heat of vaporization at the time when the cooling water partly evaporates. Therefore, the cooling water is evaporating away from the cooling tower 68 at all times. The city water and underground water used as the cooling water in the cooling tower 68 contains cations such as calcium ions, magnesium ions and dissolved silica (contained in scale). The cooling water that decreases by evaporation is constantly replenished with the city water or underground water together with cations.
Therefore, the concentration of cations contained in the cooling water gradually increases. Concretely, the electric conductivity of the city water initially supplied, which is 100 to 200 μS/cm increases to not lower than 1000 μS/cm in several days to a week. The cations coagulate to form scale, causing such problems as lowering the heat-exchanging efficiency by adhesion on the heat-exchanging surfaces of the condenser 66 and increasing the flow resistance of cooling water due to deposition on the inner surfaces of pipings through which the cooling water is circulating.
Various germs such as algae and Legionella pneumophila propagate in large amounts in the cooling water which scatters from the cooling tower together with these various germs causing such problems as may impair the health of people working around the cooling tower and may impair health of local inhabitants.
Therefore, a countermeasure has been employed for preventing the occurrence of scale by lowering the concentration of cations by diluting the cooling water with city water or ground water. However, this increases the cost of the cooling water in those districts where city water or underground water is expensive, and therefore, disadvantageously increases the cost for the maintenance and management of the air conditioners.
In establishments where the city water or underground water is not cheaply available, it has been attempted to add a chemical agent to the circulating water to control the electric conductivity of the cooling water in order to prevent the adhesion of scale on the heat-exchanging surfaces of the condenser or on the inner surfaces of pipings. However, the chemical agent must be added to the cooling water at regular intervals requiring a considerable amount of cost even when the above method is employed.
Even when the chemical agent is added to the cooling water, it is not possible to completely avoid the scale from solidly adhering on the heat-exchanging surfaces of the condenser or on the inner surfaces of pipings, and removal of the solidly adhered scale is still required, even though the interval for the removal work can be extended. Therefore, laborious work and expenditure could not be avoided.
As for the problem of propagation of algae and various germs, a countermeasure has been taken by adding a germicide to the cooling water. However, propagation of algae and various germs cannot be avoided in the long run, and algae and various germs scatter into the open air from the cooling tower together with the germicide causing air pollution.
In order to solve these problems, therefore, many kinds of purifying apparatuses have been proposed by, for example, introducing, into an electrolytic purifying vessel, an electrode unit in which electrodes are opposed to each other, introducing the cooling water into the electrolytic purifying vessel, applying positive and negative voltages to the electrodes, allowing the cations contained in the cooling water to be precipitated as scale on the surfaces of the negative electrodes, and removing the cations from the cooling water.    Patent document 1: JP-A-2001-259690    Patent document 2: JP-A-4-18982    Patent document 3: JP-A-61-181591    Patent document 4: JP-A-58-35400    Patent document 5: JP-A-2001-137891    Patent document 6: JP-A-9-103797    Patent document 7: JP-A-2001-137858    Patent document 8: JP-A-9-38668    Patent document 9: JP-A-11-114335